Refractories are materials having properties that make them suitable for use as heat-resistant barriers in high temperature applications. Unshaped refractory materials have the ability to form a joint-less lining, and are often referred to as monolithic. These materials are useful for example as linings for cupola hearths and siphons, blast furnaces, main secondary and tilting runners, and more generally as vessels or vessel spouts, ladles, tundishes, reaction chambers and troughs that contain, direct the flow of, or are suitable for facilitating industrial treatment of liquid metals and slags, or any other high temperature liquids, solids or gases.
Dense materials have a higher thermal conductivity due to their increased density. Reduction of density of monolithic refractories has therefore been employed to decrease thermal conductivity. This has previously been achieved in various ways:
Incorporation of aggregates with lower density into the refractory composition, such as high silica containing materials has been used. This however leads to a reduction in refractoriness and limits their use to low temperature applications.
Incorporation of lightweight aggregates with high porosity, such as for example vermiculite, bubble alumina, or light chamotte, has also been used. However the pore size distribution of such aggregates is such that a high proportion of pores of 100 μm or above are present, having a negative impact on the physical properties and infiltration resistance to slag and/or molten metal.
Incorporation of porosity into the refractory matrix by use of additional water or pore former such as polymeric fine particles or saw dust is also known. In this case the additional porosity leads to a drastic reduction in both infiltration resistance and compressive strength.
Also, some insulating refractories have been formed of hollow globules of alumina held by a suitable bond. However, alumina has a relatively high temperature conductivity compared to other materials such as mullite (alumina-silica) and, therefore, its insulation properties are not fully satisfactory.
U.S. Pat. No. 4,927,611 discloses a process for obtaining lightweight magnesia clinker which exhibits high apparent porosity. However, the process of obtaining such a refractory material is economically disadvantageous, in particular compared to alumina-silica refractories.
Some parameters such as density, open porosity and bulk density of the refractory have to be determined in order to assess the mechanical properties of the material. One disadvantage of conventional insulating refractory compositions with high alumina and low silica content is that, after dry pressing, adequate handling properties are lacking. This is particularly problematic in case of porous alumina refractories. A refractory material with a suitable cold compression strength is therefore desirable.
U.S. Pat. No. 4,992,397 discloses a refractory castable composition consisting of granular refractory aggregates and a cement binder comprising 55 to 80 wt.-% amorphous silica. However this refractory does not offer high heat resistance in comparison with alumina-based refractories.
U.S. Pat. No. 6,238,618 B1 discloses a method for producing porous mullite-based ceramic materials suitable for filtration purposes. This material however exhibits high permeability which is detrimental for its use as a refractory composition. Furthermore, porous lightweight aggregates are poorly bonded, which leads to low abrasion resistance.
EP 2 060 640 A1 discloses methods for the formation of lightweight porous refractory pellets formed by addition of carbonate-based reaction compounds and partner compounds, which upon pelletisation react to form gaseous CO2 which leads to the presence of open and closed pores in the final product. The pores formed have diameters of 200 μm and more, leading to pellets having low abrasive resistance and low mechanical resistance.
The size and distribution of hollow structures (such as pores) within the refractory compositions determines the degree of physical strength of the material at high temperatures. The conventional methods of producing refractory materials do not allow for a simple, economically viable and controlled porosity-formation process. The state of the art therefore represents a problem.
It is an aim of the present invention to provide a light product presenting physical properties (compressive strength, abrasion resistance, refractoriness under load etc.) that are not or only slightly decreased within acceptable limits when compared to the same non-lightened monolithic but with a lower thermal conductivity.
It is a further aim to use the said lightened monolithic for the same application than the non-lightened monolithic, such that infiltration to slag and/or molten metal is not or only slightly reduced within certain limits that do not prevent the use for the target application.